Metal halide lamps have an inner quartz arc tube containing a fill of an arc-sustaining material and surrounded by an outer glass envelope. The metal halide lamp's arc tube fill includes a rare gas for starting and a quantity of mercury. However, as compared to a mercury lamp, the metal halide lamp's emission spectrum is primarily due to the presence in the arc tube fill of one or more metal halides, usually iodides. These metal halides are responsible for a much higher luminous efficacy and better color rendering capability of the lamp output than is possible for the mercury vapor lamp.
The luminous efficacy, color rendering index and other lamp output characteristics may be varied, depending upon the particular composition of the metal halides in the arc tube. GTE's Metalarc M100/U lamp, with a NaIScI.sub.3 CsI chemistry, has a CRI (color rendering index) of 65, an initial LPW (lumens per Watt) of 85, and a 10,000 hour lifetime. In the lighting industry, these specifications are considered very good for standard lighting applications. Each chemical in the lamp is chosen to contribute specific effects to the lamp's performance. The mercury controls the current-voltage characteristics of the lamp, and the alkali metal iodides adjust the color quality, and contribute to lumen output through strong emissions. Scandium is added to the lamp as an iodide and as a pure metal. The scandium iodide improves color quality by adding a variety of lines to the color spectrum. The elemental scandium chip is used to adjust the metal/iodine ratio in the lamp and to getter oxygen impurities.
By modifying the above chemistry by the replacement of the element Cesium with Lithium to form a chemistry of NaIScI.sub.3 LiI, the resulting lamp has an improved CRI of 73 and a high LPW of 85 while still maintaining the 10,000 hour life.
U.S. Pat. No. 4,053,805 to Scholz et al relates to a red emitting metal halide arc discharge lamp utilizing a fill of mercury, scandium and lithium iodide. Lithium iodide imparts a red component to the emitted light. Problems encountered with lithium iodide as a lamp fill component, as set forth in the above patent, include lamp starting problems and electrode attack. These problems were reduced by the addition of scandium metal to the fill.
U.S. Pat. No. 4,709,184 To Keeffe et al relates to a metal halide lamp where the fill consists essentially of sodium iodide and scandium iodide in a molar ratio in the range of about 20:1 to 28:1, elemental mercury, scandium, and an inert gas. U.S. Pat. No. 4,963,790 of Robert S. White and James C. Morris describes a floating frame structure for reversing the sodium electrolysis process. U.S. Pat. No. 5,057,743 to Keeffe and Krasko relates to a metal halide lamp having a chemical fill including sodium iodide, lithium iodide, and scandium iodide. Although lamps exhibit long life, high luminous efficacy, and good color temperature, it is desirable to reduce the tendency of these lamps to reduce lumen output during extended operation.
Because of their superior efficiency and operating life, lamps utilizing a chemical fill of NaIScI.sub.3 and LiI or CsI with a scandium metal getter are highly desirable. However, it is desirable to improve lumen maintenance of these alkali metal halide lamps to discolor.